1. Field of the Invention
The present invention relates to a weighing apparatus of a type utilizing a signal processing circuit for electrically processing a load signal outputted from a load cell so that the weight of an object to be weighed can be displayed by a display unit.
2. Description of the Prior Art
Some of the currently commercially available weighing apparatuses generally utilize such an electronic signal processing circuit as shown in FIG. 9. For the discussion of the prior art believed to be relevant to the present invention, reference will now be made to FIG. 9.
The prior art weighing apparatus shown in FIG. 9 comprises a load cell 30 electrically connected with circuits formed on a main circuit wiring board 31. The load cell 30 includes four strain gauges 32 each in the form of a resistance element mounted, or otherwise provided, on a strain inducing element (not shown) which produces strains in response to a load applied thereto as a result of placement of the object to be weighed. These strain gauges 32 are electrically connected with a flexible printed circuit board (not shown) so as to form a bridge circuit 33 of a full-bridge type. The flexible printed circuit board referred to above is coupled with the main circuit board 31 through a suitable connector (not shown). The strain gauge 32 generally employed in the prior art weighing apparatuses is of a type capable of compensating for a change of a coefficient of thermal expansion of the strain inducing element so that any possible measurement error resulting from a difference in coefficient of thermal expansion between the strain inducing element and the strain gauges can be eliminated.
Of four arms of the bridge circuit 33 having the respective strain gauges 32 disposed thereon, two arms connected together through a junction c include respective adjustment elements 34 for zero-point adjustment to be accomplished during a non-loaded condition of the weighing apparatus. In certain models, the adjustment element 34 is employed on all of the four arms of the bridge circuit. In either case, each of these adjustment elements 34 includes a zero-point adjusting (offset adjusting) resistor 35 in the form of a precise resistance element used to coordinate variations in resistance value among the strain gauge 32 to adjust an output from the bridge circuit 33 to a target level, for example, a zero volt, and a zero-point temperature characteristic compensating resistor 36 in the form of a temperature sensitive resistance element connected in series with the zero-point adjusting resistor 35 for compensating for a change of the output of the bridge circuit 33 from the target level as a result of a temperature dependent drift of the bridge circuit 33.
The bridge circuit 33 has input-side junctions a and b connected with respective span temperature characteristic compensating resistors 39 which are in turn connected with a direct current power source 38 that supplies an input voltage to the junctions a and b of the bridge circuit 33. Each of the span temperature characteristic compensating resistor 39 is in the form of a temperature sensitive resistance element and is utilized to compensate for an increase of the level of the load signal which would occur when the amount of strain increases as a result of a lowering of the Young's modulus of the strain inducing element with an increase of temperature.
A voltage between output-side junctions c and d of the bridge circuit 33 is supplied to a signal processing circuit 40 provided on the main circuit board 31. This signal processing circuit 40 employed in this prior art weighing apparatus includes a differential amplifier stage 41 for amplifying the load signal outputted from the load cell 30, an analog filter for removing a noise component from the toad signal which has been amplified by the differential amplifier stage 41, an analog-to-digital converter (ADC) 43 for converting the filtered load signal into a digital load signal, and a central processing unit (CPU) 44 for calculating a measured value representative of the weight of the object to be weighed based on the digital load signal supplied from the analog-to-digital converter 43 and also for providing a display unit with the measured value in the form of a weight signal capable of being displayed.
On the other hand, the direct current power source 38 is also connected with a first voltage divider circuit 46 for providing all component elements of the signal processing circuit 40 with a first reference voltage V.sub.COM and a second voltage divider circuit 47 for providing the analog-to-digital converter 43 with a second reference voltage V.sub.REF to cause the analog-to-digital converter 43 to determine the level of an input signal supplied thereto.
The circuit configuration shown in FIG. 9 makes use of a number of resistance elements in, for example, the four strain gauges 42 and the two voltage divider circuits 46 and 47 and, thus, the number of component parts employed therein is relatively great. Also, since the resistance of each of the strain gauges 32 of a kind employed in the weighing apparatus is generally relatively low, the bridge circuit 33 consumes a relatively large amount of electric power. Therefore, in the case of the weighing apparatus powered by a battery, the battery is apt to run out quickly.
In view of the problems discussed above, the use has been suggested of such a bridge circuit 33A as shown in FIG. 10. The bridge circuit 33A shown in FIG. 10 is so designed that a difference between a voltage appearing at the junction d between the strain gauges 32 and the first reference voltage V.sub.COM may be utilized as a load signal. This bridge circuit 33A has been considered advantageous in that, when each of the voltage divider circuits 46 and 47 is chosen to have a relatively high resistance, the amount of the electric power consumed by the bridge circuit 33A can be considerably reduced. However, this configuration of the bridge circuit 33A has been found having the following two problems.
In the first place, unless the span temperature characteristic compensating resistors 39 have the same resistance and the same temperature characteristic, the voltage at the junction d from which the load signal can be taken out fluctuates with change in temperature, that is, becomes unstable with change in temperature.
Secondly, since the level of the load signal is represented by a difference between the voltage at the junction d between the strain gauges 32 and the first reference voltage V.sub.COM, it is necessary to compensate totally for the temperature characteristic of both of the voltage divider circuit 46 and the strain gauges 32 in order to obtain the load signal stable relative to the change in temperature. And, since the strain gauges 32 are fitted to the load cell 30 on one hand and, on the other hand, the voltage divider circuit 46 is fitted to the main circuit board 31, an adjustment necessary to compensate for temperature-dependent variation must be carried out while the load cell 30 and the main circuit board 3I are assembled together. Therefore, the adjustment tends to be complicated and time-consuming. In addition, since the load cell 30 and the main circuit board 31 once they have been adjusted in the assembled condition have no compatibility with any other load cell or main circuit board and must be employed always in a paired fashion in a particular weighing apparatus, a replacement of one of the load cell 30 and the main circuit board 31 requires a corresponding replacement of the other of the load cell 30 and the main circuit board 31. Thus, when one of the load cell and the main circuit board both employed in a particular weighing apparatus is required to be replaced, the both must be replaced with a pair of the load cell and the main circuit board which have been separately adjusted, resulting in a waste of component parts.
To substantially alleviate the foregoing two problems, the inventors of the present invention have suggested, in their U.S. Pat. No. 4,951,765, such a circuit configuration as shown in FIG. 11 in which the use of the span temperature characteristic compensating resistors 39 have been dispensed with. According to the circuit configuration shown in FIG. 11, the use has been made of a temperature sensitive resistor 39A for the purpose of automatically adjusting the gain of the differential amplifier stage 41 in accordance with change in temperature so that a change of the load signal due to the change in temperature can be compensated for by the automatic adjustment of the gain of the differential amplifier stage 41. The circuit configuration shown in FIG. 11 is effective to resolve the first mentioned problem associated with the load signal becoming unstable with change in temperature, but it has been found that the second mentioned problem associated with difficulty in adjustment and the waste of component parts remains unsolved.